METAR (Aviation Routine Weather Report)—a weather observer's interpretation of the weather conditions at a given site and time. Two types: Routine (issued every hour) and SPECI (Special Observation—issued at any time to update the METAR for rapidly changing weather conditions, aircraft mishaps, or other critical information).

Basic elements of a METAR: Type of report (routine or SPECI); 4-letter station identifier; Date and time of report (6-digits appended with z—first 2 digits are date followed by 2 digits for hour and 2 digits for minutes); Modifier (AUTO); Wind (5 digits—first 3 are direction wind is from in tens of degrees referenced to true north—directions less than 100 degrees preceded with a zero—next two digits are avg. speed in knots); Visibility (in statute miles followed by a space and fractions of statute miles, then the letters SM); Runway Visual Range (RVR); Weather phenomena (broken into two categories—qualifiers and weather phenomena); Sky condition (Amount/Height/Type or Indefinite Ceiling/Height—Vertical Visibility); Temperature/Dewpoint (Two-digit form in whole degrees separated by a / ) [temperatures below zero are prefixed with “M"]; Altimeter (4 digits prefixed with "A"); Remarks (RMK). 

Example (See picture at top):

METAR indicates that the following is a standard hourly observation.

KTTN is the ICAO identifier for the Trenton-Mercer Airport.

051853Z indicates the day of the month is the 5th and the time of day is 1853 Zulu/UTC, 6:53PM GMT, or 1:53PM Eastern Standard Time.

04011KT indicates the wind is from 040° true (north east) at 11 knots (20 km/h; 13 mph).

1/2SM indicates the prevailing visibility is 1/2 mi (800 m) SM = statute mile.

VCTS indicates a thunderstorm (TS) in the vicinity (VC), which means from 5–10 mi (8–16 km).

SN indicates snow is falling at a moderate intensity; a preceding plus or minus sign (+/-) indicates heavy or light precipitation. Without a +/- sign, moderate precipitation is assumed.

FZFG indicates the presence of freezing fog.

BKN003 OVC010 indicates a broken (5/8  to 7/8 of the sky covered) cloud layer at 300 ft (91 m) above ground level (AGL) and an overcast (8/8 of the sky covered) layer at 1,000 ft (300 m).

M02/M02 indicates the temperature is −2 °C (28 °F) and the dew point is −2 °C (28 °F). An M in front of the number indicates a negative Celsius temperature/dew point (“minus”).

A3006 indicates the altimeter setting is 30.06 inHg (1,018 hPa).

RMK indicates the remarks section follows.

AO2 indicates that the station is automated with a precipitation discriminator (rain/snow) sensor. (Stations that aren't equipped with a rain/snow sensor are designated AO1).

TSB40 indicates the thunderstorm began at 40 minutes past the hour at 1840 Zulu/UTC, 6:40 p.m. GMT, or 1:40 p.m. Eastern Standard Time.

SLP176 indicates the current barometric pressure extrapolated to sea level is 1,017.6 hPa (30.05 inHg).

P0002 indicates that 0.02 inches (0.5 mm) of liquid-equivalent precipitation accumulated during the last hour.

T10171017 is a breakdown of the temperature and dew point in eight digits separated into two groups of four. The first four digits (1017) indicate the temperature. The first digit (1) designates above or below zero Celsius (0=above zero 1=below zero). The next three digits in the group "017" give the temperature in degrees and tenths of a degree Celsius, −1.7 °C (28.9 °F). The last four digits "1017" indicate the dew point, −1.7 °C (28.9 °F). Note: ASOS software, as of this update, uses whole degrees in °F to compute the °C values in this group.

= indicates the end of the METAR.